Schematic diagram of the general structure of a set-top box
The digital set-top box consists of several important modules:
a. Modulation and demodulation module
The digital set-top box operates in the state of the cable network, which adopts analog transmission, so it must be modulated and demodulated in order to be transmitted on the analog channel, and the modulator is the key component of the system. The modem is a key component of the system, which is technically similar to the principle of telephone modem nowadays, but adopts a higher modulation method, and 64QAM or 256QAM is mostly used for downlink, and 64QAM is adopted as the standard modulation method in DVB-C (Digital Video Broadcast by Cable) and DAVIC, and the MC92305QAM demodulator of Motorola is used as the standard modulation method. In DVB-C (Digital Video Broadcast by Cable) and DAVIC, 64QAM is used as the standard modulation method, for example, Motorola's MC92305QAM demodulation chip, in 7M analog bandwidth, the rate of the digital signal can reach 42Mbit/S with 64QAM modulation, and uplinking is currently used in two ways, one is to use the telephone line as the uplink channel, and the other is to use the uplink channel of the bi-directional HFC network, and the modulation scheme of the HFC network is used in the QPSK mode. The QAM or QPSK modulator modulates the MPEG format data stream in a standard PAL channel and sends it out together with other video modulation signals.
b. Codec Module
Due to the use of analog channels, forward error correction coding is indispensable in order to ensure the reliability of data transmission and low BER, DVB adopts Reed Solomon coding, which is a type of multicode BCH code with strong error correction capability.
c. MPEG II decoding module
The core of digital set-top box is digital video technology, MPEG II decoding module can be called the core module other than the CPU, MPEG II digital transmission using interleaving coding, first of all, you need to de-interleave the stream, the separation of the video, audio and data streams, and decoding of the video code. After the above steps, the MPEG II stream becomes a video (CCIR656 format) and audio digital signal. MPEG multiplexer will be the program stream, data stream composite together to 188 bytes as a frame of MPEG2 data format sent to the RF modulator and provide electronic program list (EPG).
d. Digital Video Encoder and Audio DAC
The "peripherals" of a digital set-top box are the TV and sound system, and the digital audio and video signals must be converted to analog audio and video signals. ITU601, 656 standard 4:2:2 parallel video data into PAL or NTSC format video, S-Video, Y/Cb/Cr or R/G/B, extended VBI (Vertical Blanking Interval) information input for displaying graphic information. Audio DACs are very common and are not described here. Modern audio systems support Dolby Pro Logic and Lucasfilm Home THX home theater systems, the need for digital audio signal processing chip in the audio DAC before the digital audio signal processing, there are dedicated processing chip and digital signal processing chip DSP for processing two ways.
e. Graphics control module
and video graphics mixer digital set-top box on the other hand, need to provide complex computer-like graphics or even three-dimensional graphics, the system needs to have a powerful two-dimensional and three-dimensional graphics accelerator chip, and the need for graphics and video signals can be mixed to superimpose the display, such a function is similar to the existing subtitle machine.
f. Media Access Control Module (MAC)
A digital set-top box can be regarded as a Cable Modem in network communication, and an IP router receives the on-demand signals from the user via PSTN (Public Switched Telephone Network) and transmits them to the Internet server/router. At present, the main reference DAVIC and IEEE 802.14 standards, IEEE802.14 advocate the use of ATM and 802.14 MAC access layer defined on the ATM layer, DAVIC at the same time defined the Ethernet and ATM standards.
h. Erasable DVD
Existing digital set-top boxes work entirely in a networked environment, and can only use smart cards to store information such as personal account numbers and decoding numbers, but do not have local big data storage, and all the information needs to be obtained from the Internet. The disadvantage of CD-ROM is that the information cannot be updated. The disadvantage of CD-ROM is that the information can not be updated. The use of rewritable DVD can not only use the huge information on the CD-ROM, but also more effectively update the data through the Internet, so that the data on the DVD disk is kept up to date, and the DVD should become an important backup module of the digital set-top box.
i. Scrambling Module and Copyright Protection Module
In cable TV operation, pay TV is a major business, which requires digital set-top boxes to have the scrambling function of TV signals. Due to the use of digital signals, scrambling is easier than analog signals and the density is higher, and there is a definition of the scrambling function by DVB. On the other hand, the use of digital signals increases the difficulty of copyright protection, and Macrovision generator is currently used for the protection of moving images.
How a set-top box works:
The various modules of a set-top box work together in high-speed access to the Internet. A user first requests a URL by sending a command that eventually reaches the modem*** rack of the Internet service provider during a single transaction using the PPP peer-to-peer protocol. Then, through the dynamic assignment method, that user obtains the IP address to be used in this transaction and sends the request to the Internet.When the Internet content is found, it is then sent to the Internet Service Provider (ISP), whose routers are responsible for delivering it to the cable television network and finally back to the subscriber's set-top box. At the cable TV set-top box, the information content is intercepted. The set-top box fulfills the role of a gateway between the TV set and the cable network. It is tasked with managing IP traffic and has the ability to control subscriber access to and from the network Once an IP packet arrives at the set-top box, separates the video signal from that packet, decodes the data in it, and sends it to the browser ready to be displayed on the TV set.
Because Cable Modem requires users to configure a computer to access the Internet, affecting the expansion of the user layer, and the use of set-top boxes do not need a computer, a TV is enough, and thus the market outlook for set-top boxes may be favorable. The transition of information users from enterprises to families, the transition of network bandwidth from narrowband to broadband, the transition of user access devices from PCs to TVs with set-top boxes (STBs), the transition of user interfaces from Windows to TV remote key interfaces, and the transition of information content from serving enterprises to serving people's lives are the general direction of network service development, and set-top boxes (STBs) are obviously a stage in this general direction. stage.